I suggest you just keep the day open. Depending how the rest of the trip goes, you might want to hit a park to do something you missed, shop for that thing you forgot to get, or if you have hoppers, hit 2 or more parks re-riding favorites.
If your last night was a super late one, then sleeping in and laying by the pool might sound best.
It can also be a good day to hit a water park if you can't fit it in earlier.
The resort will hold your carry on luggage and check your main luggage in for your flight. Just keep a small bag with you with swimming stuff (even it you don't "plan" on swimming). The bathrooms near the pool are fine for changing before picking up ME - If I remember right they even have showers with soap.